The present invention relates to a method for RARE magnetic resonance imaging comprising slice selective excitation of two or more slices and of one or more nuclei, followed by refocusing of these slices and application of gradient pulses which cause a diversion of the signal into spin echoes and stimulated echoes.
Such nuclear magnetic resonance signal is frequently measured by means of the spin echo method known from [1].
After a period te/2, the excited magnetization is refocused with an RF pulse and a spin echo is formed after a further time period te/2. The signal maximum is achieved if the flip angle of the refocusing pulse is exactly 180 degree. To increase efficiency of data acquisition, long echo trains can be used, a technique called RARE [2]. To reduce RF energy deposition from a long train of refocusing pulses, the hyperecho method may be used [3].
In 2003, the simultaneous image refocusing method (SIR) was introduced [4]. As described in the patent, with this technique, total imaging time is reduced when any “preparation” is used. Preparation here refers to RF pulses, gradient lobes and delays which, for example, are used to encode for a particular contrast. It can also include phase encoding, and crusher gradients. The time is reduced because these components are shared between two or more slices. However, when applying SIR to a RARE sequence, the CPMG conditions are not met, and to prevent interference from stimulated echoes, special spoiler schemes are required to spoil incorrectly timed echoes [5].
U.S. Pat. No. 5,237,273 describes a spin-echo sequence for a multi-slice NMR scan from three separate slices. The major disadvantage of this method is its relatively long acquisition time.
It is a major object of the present invention to increase and stabilize the obtainable signal.